An engine of this type is described in U.S. Pat. No. 3,280,560. Essentially it is a by-pass type engine which has a conventional axial type discharge nozzle for normal forward propulsive thrust in wing-borne flight. For jet-borne flight the by-pass and exhaust flows are selectively diverted. The cold by-pass flow is discharged through a pair of forward mounted vectorable nozzles on port and starboard sides of the engine. The hot turbine exhaust is also blocked by a diverter valve, in this case of the "clamshell" type. The valve members divert the axial flow to discharge through rearward mounted, also port and starboard, vectorable nozzles. The clamshell valves each comprise a fragment of a cylinder with oblique upstream and downstream edges. The valve members are mounted on vertical trunnions so they can be swung between a first position in which each blocks a side exit aperture in the jet pipe wall and a second position in which they come together to block flow to the axial nozzle.
A drawback with this type of arrangement arises due to area mismatching. During transition between operating modes the total effective outlet area, immediately downstream of the turbine section, undergoes a substantial although temporary increase even if the jet pipe and side outlet areas are closely matched. This occurs as the clamshell valves begin to uncover the side outlets and to block the jet pipe. Unfortunately the initial effect of the latter is overwhelmed by initial effect of opening the side exits.
A different type of valve arrangement which may be adapted to divert gas flows is disclosed in U.S. Pat. No. 4,587,803. The engine described therein has a tandem fan arrangement with a changeover valve between the fans to selectively divert flow from the front fan either through an axial duct leading to the second fan or through side exits exhausting to atmosphere. A sleeve valve blocks or uncovers the side exits by moving axially. Pivoted to the sleeve valve are a plurality of valve segments which are restrained by a fixed length link thereby causing the segments to pivot inwardly to block the axial duct as the sleeve is translated rearwards. This arrangement may be adapted for hot exhaust gas flows by the use of suitable materials to construct the clamshell valve members described above.
Both valve arrangement suffer from the same drawback of area mismatch. The present invention has for its primary objective to provide a solution to this problem.